Thick oil buildup around gearbox hmmm rear main seal maybe? Clutch doesn't slip does it?There should be a transmission dipstick to are levels fine In both? Get the oil on your finger and smell it. Gear oil smells turrible and engine oil not that bad unless filthy. Any car that sits I would give high mileage oil for 2k miles then switch to rotella as the high mileage will swell the seals.
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I know most people with wrx's reccomend replacing the rear main when doing the clutch, so I think they may be more prone to failure on Turbo Subies especially if they have time to sit and shrivel up, but as you can see from the picture both are in the same area so pulling the engine will verify either leak to you.